The present invention relates to a power adjustment method for adjusting write power, an information write method with the power adjustment conducted, and an optical information write medium which retains information to be used in the adjustment of the write power.
At the present time, the CD (Compact Disc), DVD (Digital Versatile Disc), BD (Blu-ray Disc) and so on are put on the market and spread as optical discs which are optical information write media. In these optical discs, there are various kinds such as the ROM (Read Only Memory) type, R (Recordable) type which is write-once type, and RE (Rewritable) type. Information writing and reading in an optical disc are conducted by irradiating the optical disc with laser light in an optical disc which is an optical information write apparatus. Information writing is conducted by changing irradiation power of laser light and forming marks changed in optical characteristics on a data layer of the optical disc. Information reading is conducted by irradiating the data layer of the optical disc with laser light having power lower than that used when forming the marks and detecting a difference in reflected light quantity every irradiation position.
The optimum write power used when writing information differs depending upon the classification of the optical disc and optical disc fabricating maker. It is necessary to set the optimum write power in the drive according to the kind of the optical disc. Even if optical discs are the same in kind, however, the optimum write power differs every optical disc because of the fabrication dispersion. In some cases, optimum writing cannot be implemented if the same write power is used. Even if the same optical disc is used, optimum writing cannot be implemented with the same setting power in some cases because of power dispersion of drives. Therefore, each drive has a configuration in which trial writing is conducted in a predetermined area and optimum write power is adjusted depending upon the optical disc prior to writing in the optical disc.
As a write power adjustment method using trial writing, a method using a relation between write power and the modulation is proposed for, for example, BD-RE discs. This is disclosed in Non-Patent Document 1 which is a document of specifications for BD-RE.
According to this method, optimum write power Pw-opt is calculated by using a relation between write power Pw and a modulation M, indicated write power Pind, a target coefficient value κ, and a coefficient ρ which are predetermined parameters. These parameters are previously written in a management area of the optical disc. First, a predetermined signal is written in a predetermined area of the optical disc by using write power Pw,m (where m is an integer) of a plurality of kinds in the vicinity of the indicated write power Pind, and the written signal is read. As a result, a modulation M,m, which is a value obtained by dividing an amplitude of a read signal by a higher envelope level, is acquired so as to be associated with the write power Pw,m. Then, a relation between an evaluated value M,m×Pw,m and the write power Pw,m is subjected to straight line approximation over a predetermined power range having Pw,m as its center. A value of the write power Pw,m obtained when the evaluated value M×Pw becomes zero is calculated as a write power threshold Pthr,m. In a relation between a target write power Ptarget, m=κ×Pthr,m obtained by multiplying the calculated Pthr,m by the target coefficient value κ and the write power Pw,m, write power Pw satisfying the relation Ptarget=Pw is determined as optimum target write power Ptarget-opt, and a value obtained by multiplying the optimum target write power Ptarget-opt by the coefficient ρ is determined as optimum write power Pw-opt.
According to the method described heretofore, it is possible for each drive to set optimum write power for each optical disc and optimum writing can be implemented. Since the write power adjustment described in Non-Patent Document 1 and described heretofore is a scheme executed by using the target coefficient value κ, this write power adjustment method is hereafter referred to as κ scheme.
A write power adjustment method which uses the relation between the write power and the modulation and which differs from the above-described method is disclosed in Patent Document 1.
According to this method, optimum write power Pw-opt is calculated by using a ratio α=Pw-opt/Pth between optimum write power Pw-opt and a write power threshold Pth which are predetermined parameters. First, a predetermined signal is written in a predetermined area of an optical disc by using write power Pw,m (where m is an integer) of a plurality of kinds, and the written signal is read. As a result, a modulation M,m associated with each write power Pw,m is acquired. Then, write power correction values Pc,n (where n is an integer) of a plurality of kinds are set. Pc,n that makes a relation between an evaluated value M,m×(Pw,m−Pc,n) and corrected write power (Pw,m−Pc,n) closest to a straight line is determined as an optimum write power correction value Pc-opt. When straight line approximation is applied to a relation between an evaluated value M,m×(Pw,m−Pc-opt) and corrected write power (Pw,m−Pc-opt), a value of the corrected write power (Pw−Pc-opt) that makes an evaluated value M×(Pw−Pc-opt) equal to zero is calculated as a corrected write power threshold Pth′. A write power threshold Pth=Pth′+Pc-opt is calculated by using the calculated Pth′. Optimum write power Pw-opt=α×Pth is determined by multiplying the calculated Pth by a coefficient α.
If this method is used, then the optimum read power can be set with high precision and optimum writing can be implemented with respect to an optical disc having a curvilinear relation between an evaluated value M×Pw and write power Pw and a medium having a modulation which changes uniquely (having a magnification which changes uniquely at all measurement points) according to the state of an optical disc and the read condition.
Here, the Patent Document 1 in this specification is JP-A-2007-334922.
Further, the Non-Patent Document 1 in this specification is “System Description Blu-ray Disc Rewritable Format,” Part 1, Basic Format Specifications, Version 1.02, Blu-ray Disc Founders, June 2004, pp. 197-200.